SOILS..(U) mhhhhhl--mIl mmhhhhhhhhhhhl hlflflflflflflfll ... · of soils at the interface of piles that support offshore structures. These soils are sub- ... CHAPTER 7. RECOMMENDATIONS

AD-Ai 3 332 DEVELOPMENT OF A LABORATORY TEST TO IDENTIFY THE SCOUR 1/1POTENTIAL OF SOILS..(U) TEXAS UNIV AT AUSTINGEOTECHNICAL ENGINEERING CENTER S WANG ET AL. FEB 88

UNCLASSIFIED WES/ P/ OL- B 9-3B 3 -82-C- B14 F/G 8/S1 NL1Mhhhmhhhhhhhmhhhhhl--mIlmmhhhhhhhhhhhl

I hlflflflflflflflllflflflflflflflllllllI/ |l|//i

11111 - L 32

L

111.2= LA IB

(JMICROCOPY RESOLUTION TEST CHARTNATIONAL BUREAU OF STANDARDS 1963-A

%N

nMISCELLANEOUS PAPER GL-88-3

DEVELOPMENT OF A LABORATORY TESTTO IDENTIFY THE SCOUR POTENTIAL OF SOILSAT PILES SUPPORTING OFFSHORE STRUCTURES

N by

() Shin-Tower Wang, Lymon C. Reese

Geotechnical Engineering CenterBureau of Engineering Research

The University of Texas at AustinAustin, Texas 78712

" DTIC'S ':LECTE

DTD

February 1988

Reprint of Geotechnical Engineering Report GR83-2

'; Approved For Public Release Distribution Unlimited

+IPrepared for US Army Engineer Waterways Experiment Station

LABORATORY PO Box 631, Vicksburg, Mississippi 39180-0631

Under Contract No. DACW39-82-C-0014 u 3 6

1832

SECURITY CLASSIFICTION F THIS PAGEForm Approved

REPORT DOCUMENTATION PAGE OMBNo 0704.01"I I EaP Date JunJO0. 1986

Ia. REPORT SECURITY CLASSIFICATION lb. RESTRICTIVE MARKINGSUnclassi fied ____ ____ ____ __/AVAILABILITYOFREPORT

2a. SECURITY CLASSIFICATION AUTHORITY 3. OISTRIBUTIONAVAILAiLITY OF REPORTApproved for public release; distribution

2b. DECLASSIFICATION/OWNGRADING SCHEDULE unlimited.

4. PERFORMING ORGANIZATION REPORT NUMBER(S) 5. MONITORING ORGANIZATION REPORT NUMBER(S)

Geotechnical Engineering Report GR83-2 Miscellaneous Paper GL-88-3

Be. NAME OF PERFORMING ORGANIZATION 6b. OFFICE SYMBOL Yo. NAME OF MONITORING ORGANIZATION

See reverse Geotechnical Laboratory, US Army EngineerWaterways Experiment Station

6C. ADORESS (CRY, State. and ZIPCode) 7b. ADDRESS(City, State. and ZWCode)

Po Box 631Austin, TX 78712 Vicksburg, MS 39180-0631

84. NAME OF FUNDING/SPONSORING Bb. OFFICE SYMBOL 9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBERORGANIZATION (if appikableJSee reverse

8c. ADDRESS (My. State. and ZIPCode) 10. SOURCE OF FUNDING NUMBERSPROGRAM PROJECT TASK [WORK UNIT

See reverse ELEMENT NO. INO. N. ASSN NO.

11. TITLE (ftdude StOu10 O.N/1flation)Development of a Laboratory Test to Identify the Scour Potential of Soils at Piles Support-ing Offshore Structures

12. PERSONAL AUTHORS)Wang, Shin-Tower; Reese. Lvmon C.

13a. TYPE OF REPORT 13b. TIME COVERED 14. DATE OF REPORT (Year, MO¢N, Day) IS. PAGE COUNTFinal re ort FROM _______TO February 1988 83

16. SUPPLEMENTARY NOTATIONAvailable from National Technical Information Service, 5285 Port Royal Road, Springfield,VA 22161.

17. COSATI CODES 18. SUBJECT TERMS (Continue on reverse if necewry and identiy by block number)FIELD GROUP SUB-GROUP Cyclic loads Marine sediments Scour

Lateral loads Piles

19. ABSTRACT (Continue on reverse if necenty and identify by block number)-This report describes the development of a test to investigate the scour potential

of soils at the interface of piles that support offshore structures. These soils are sub-jected to cyclic loading due to wave action. The test is called the Pile-Soil Scour Po-tential Test (PSSP). The mechanism of scour around the pile-soil interface is discussed.Due to the complicated interactions between pile, soil, and fluid at the interface, thisstudy is primarily directed toward en experimental approach. In addition to the use of thePSSP test, the existing pinhole test was used to study the scour potential of various soils.

A series of PSSP tests were conducted for examining the scour resistance of soils.The influences of variables such as soil properties, loading frequency, pile diameter, andduration in the scour process were studied by use of the PSSP test.

The most significant findings of these tests are analyzed. A method of predictingthe scour potential is suggested and further studies are recommended to improve the method.

20. DISTRIBUTION/AVAILABILITY OF ABSTRACT 21 ABSTRACT SECURITY CLASSIFICATIONQ UNCLASSIFIEDIUNLIMITED C3 SAME AS RPT. Q DTIC USERS Unclassified

22a. NAME OF RESPONSIBLE INDIVIDUAL 22b TELEPHONE (Include Area Code) 22c. OFFICE SYMBOL

Do FORM 1473.84 MAR 83 APR edition may be used until exhausted. SECURITY CLASSIFICATION OF THIS PAGEAll other editions are obsolete. Unclassified

Unclassi fiedqgimTY oiL.AgiICATw OF vNIn PAGE

6a. NAME OF PERFORMING ORGANIZATION (Continued).

Geotechnical Engineering CenterBureau of Engineering ResearchThe University of Texas at Austin

8a. NAME OF FUNDING/SPONSORING ORGANIZATION (Continued).

Minerals Management ServiceUS Department of InteriorReston, VA 22090

US Army Engineer Waterways Experiment StationPO Box 631Vicksburg, MS 39180-0631

UnclassifiedSZCUMIY C6AUSItCATewO orI PAGE1

PREFACE

This study was performed by the Geotechnical Engineering Center, Bureau

of Engineering Research, The University of Texas at Austin, under contract to

the US Army Engineer Waterways Experiment Station (WES), Vicksburg, Missis-

sippi, for the Minerals Management Service, US Department of Interior. The

study was performed under Contract No. DACW 39-82-C-0014.

This report was prepared by Mr. Shin-Tower Wang and Dr. Lymon C. Reese,

University of Texas at Austin, and reviewed by Mr. Gerald B. Mitchell, Chief,

Engineering Group, Soil Mechanics Division (SMD), Geotechnical Laboratory (GL),

WES. General supervision was provided by Mr. Clifford L. McAnear, Chief, SMD,

and Dr. William F. Marcuson III, Chief, GL.

COL Dwayne G. Lee is Commander and Director of WES. Dr. Robert W.

Whalin is Technical Director.

Ace siion ior jNTIS CRAW&I

DTIC "7AU:i:j',v;-:'.,ed ]

B. . . ........... ...... ..................

,i-iI I:{,cl i

0 s:

TABLE OF CONTENTS

Page

PREFACE ±

LIST OF TABLES

LIST OF FIGURES iv

CHAPTER 1. INTRODUCTION 1

CHAPTER 2. THE INFLUENCE OF SCOUR ON PILE FOUNDATION DESIGN 4

CHAPTER 3. THE MECHANISM OF SCOUR AROUND THE PILE-SOIL INTERFACE 9

CHAPTER 4. THE RESEARCH APPARATUS AND PROCEDURES 10

4.1 Pile-Soil Scour Potential (PSSP) Test 10

4.2 Pinhole Test 15

CHAPTER 5. RESULTS AND ANALYSES 23

5.1 Measurement of Resistance of Soil to Interface Scour 23

5.2 Measurement of the Influence of Various Factors on Scour 26Resistance

5.2.1 Effect of Soil Properties on Scour Resistance 32

5.2.2 Frequency of Pile Movement 36

5.2.3 Pile Dimensions 42

5.3 Equilibrium Stage of Scour Around the Pile-Soil Interface 42

5.4 Comparison of Results from Pinhole Test and from PSSP Test 49

5.5 Data Interpretation 51

CHAPTER 6. RESULTS FROM TESTING NATURAL SOILS 60

CHAPTER 7. RECOMMENDATIONS 65

CHAPTER 8. SUMMARY AND CONCLUSIONS 66

REFERENCES 68

N69

APPEND4

LIST OF TABLES

Table Title Page

5.1 Summary of soil properties 24

5.2 Summary of test results from pinhole test andPSSP test 50

5.3 Summary of scour indexes of samples 58

6.1 Summary of soil properties for Manor, North Sea,and Sabine Clay 61

a

IiiJ

LIST OF FIGURES

Figure Title Page

1.1 Concepts related to scour around pile foundation 2

2.1 Characteristic shapes of the p-y curves for soft clay 5below the water surface

2.2 Characteristic shapes of the p-y curves for stiff clay 6below the water surface

2.3 Relationships of soil resistance, pile deflection and 7

scour quantity

4.1 Apparatus used in pile-soil scour potential test (PSSP Test) 11

4.2 Details of components of PSSP test 13

4.3 Apparatus for pinhole test 16

4.4 Section through pinhole test specimen 17

4.5 Sequence of testing and classification of test results 22for pinhole test

5.1 Comparison of scour resistance of various samples 25

5.2 Gradation curves of sample SN-l and sample from Skyway Bridge 27

5.3 View of Manor Clay during testing, 10 cycles 28



5.6 View of Manor Clay during testing, 600 cycles 29 £

5.7 View of scour gap after testing 30

5.8 View of non-scour gap after testing 30

5.9 Gradation curves of scoured soils and scour-resistant soils 31

5.10 View of 6-in.-diameter undisturbed Manor Clay 33

5.11 Influence of sample disturbance on scour resistance of 34Manor Clay

ivU

5.12 Influence of soil density on scour resistance 35

5.13 Influence of pile frequency on scour resistance of Manor Clay 37

5.14 Influence of the humber of cycles on scour resistance of 38Manor Clay

5.15 Influence of fluid velocity on scour resistance of Manor Clay 41

5.16 Influence of pile diameter on scour resistance of Manor Clay 43

5.17 Relationship between scour rate and duration of testing of 45Manor Clay

5.18 Relationship between scour rate and duration of testing of 47Manor Clay under constant pile deflection (1.6 mm) andpile frequency (90 RPM)

5.19 Relationship between scour rate and duration of testing of 48

Manor Clay under constant pile frequency (60 RPM)

5.20 Relationship of pile frequency and dry weight of scoured soil 53

5.21 Relationship of duration and dry weight of scoured soil 54

5.22 Relationship of pile deflection and dry weight of scoured soil 55

5.23 Comparison of experimental data and fitting equation for 56Manor Clay at constant pile deflection (1.6 -,-)

5.24 Comparison of experimental data and fitting equation for 57Manor Clay at constant pile frequency (60 RPM)

6.1 Gradation curves of natural soils 62

6.2 Comparison of scour resistance of Manor Clay, North Sea Clay, 63and Sabine Clay

vSIVrr

CHAPTER 1. INTRODUCTION

Considerable evidence is available to show that reduced capacity

under lateral load of piles supporting offshore structures can be due to

the effects of piping and erosion by fluid.

The scour 1,2,3 around offshore structures on the sea floor caused

by the erosive action of both oscillatory waves and unidirectional water

currents has been studied for a long time. Soils around the foundations

of these structures are scoured and carried away by currents thus reducing

considerably the designed lateral capacity. Many factors controlling the

scour phenomenon on the sea floor such as current velocity, wave character-

istics, water depth, and shape of foundation have been examined in the

past 10 years.

In addition to the scour of the sea floor around an offshore struc-

ture, the pumping force around the pile-soil interface caused by the cyclic

movement of the pile presents another kind of scour phenomenon which has

been less understood to date. A scour gap around the pile is formed after

continual cyclic movement (shown in Fig. 1.1). The scour around a pile

will reduce the lateral capacity of the pile, thus reducing the factor of

safety. If the loss of resistance due to scour had not been considered in

design, it is not beyond reason that the scour could cause a pile to fail

leading to a failure of the structure.

The purpose of the study presented in this report was to develop

a test that can be used to examine the scour potential of the soils around

piles that support offshore structures. In order to provide a background

for the experimental studies that were done, some description will be given1J

2

VERTICAL LOAD

SCOUR (EROSION)ON SURFACE

LATERAL LOAD ---

ONCOFLOW

SPARTION4T ON

'" " AKE PLUME

PRIMAkRY'" ": " . ",..,VORTEX

I ..

1 EROSION AT THE

i PILE-SOIL INTERFACE

Fc

Fig. 1.1 Concepts related to scour around pile foundation

• 0

3

concerning the formation of scour holes around a pile foundation in the

marine environment. Because the fluid flow is so complex at the pile-

soil interface, this study is primarily directed toward an experimental

rather than a theoretical approach.

The above discussion is general and is not specific with regard to

soil type. Unidirectional water currents will scour non-cohesive, fine-

grained sand to a considerable extent. There are cases where several feet

of sand have been eroded around an offshore structure. Unidirectional

currents will scour clays only to a limited extent, if at all. Erosion at

the pile-soil interface is associated only with soils where a space will

open or with cohesive soils. Cohesionless soils will collapse around the

deflected pile or will move with the pile as it is deflected.

The results are aimed at providing an engineer in offshore design

information such that the scour potential of the soil at a particular off-

shore site can be estimated.

This is the first of two reports that will deal with the loss of

resistance of laterally loaded piles due to cyclic loading. In the second

report, the loss of resistance due to two phenomena are studied. In addition

to the loss of resistance due to scour, the loss of resistance due to cyclic

deformations within the soil mass is examined. These two phenomena are used

as a basis for explaining results of full-scale cyclic lateral-loading tests.

PP.4

• I

CHAPTER 2. THE INFLUENCE OF SCOUR ON PILE FOUNDATION DESIGN

The deflection of a pile under lateral load is dependent on the soil

reaction against the pile. The response of the soil surrounding the laterally

4loaded pile can be described in terms of p-y curves which relate the soil

resistance to the pile deflection at various depths. Generally, these curves

are nonlinear and depend on several parameters such as shear strength, pile

geometry, and loading conditions.

Because of the complex nature of the soil behavior around a laterally

loaded pile, the major field experiments have led to the development of the

current criteria for p-y curves. If the p-y curves for soft clay 5 and stiff

clay 6 below the water surface shown in Figs. 2.1 and 2.2, respectively, are

examined, it can be seen that there is a marked deterioration of soil response

during cyclic loading. The decrease of soil response is caused by soil dis-

turbance and the scour around the pile. As noted earlier, the scour occurs

in cohesive soils and is thought to be one of the important factors influencing

the loss of soil resistance.

As shown in Fig. 2.3a, the soil acts almost elastically to a given

deflection and after that deflection a pile in clay will experience reduced

resistance due both to scour and loss of resistance of the clay due to repeated

loadings. Figure 2.3b shows that the loss of resistance due to scour is

associated with the amount of soil that is removed around a pile as a result

of the cyclic loading.

The current criteria for p-y curves for cyclic loading are based on

limited data. These criteria may be conservative for some scour-resistant

4

5

LC

P

010 10 so

Y/Y5 0

(a) STATIC LOADING

1.00-

0.72--- FOR x ,-xr (de~h where f low

P

I i

Y/I Y50

(b) CYCLIC LOADING

Fig. 2.1 Characteristic shapes of the p-y curves for soft clay below the

water surface (after Matlock, 1970)

6

/OFFSET =0.055 P Y -A gY~ 50-

.Y5

SIntersection of

cr _ j_ P initial straight lie

and first parabolicw~. STATIC " ioF- STTI portions

OAgYo 6 AgY50 18AgYo

DEFLECTION, Y, in

0i: P=A(,. I Y- 0-.45YplC45Yp

WA Y5

kx CYCLIC-J

o 046Yp o.5O, ISYp

DEFLECTION,Y, in

0

Fig. 2.2 Characteristic shapes of thep-y curves for stiff clay below

the water surface (after Reese et al., 1975)

0

- . N.. . .. *

7 -s

Static Loading

* /loss of resistance due to4 repeated loading and scour-S

ing at pile-soil interface

-4CyclicLadn"40

Pile Deflection (a)

0U

0

,

Pile Deflection (b)

I

Fig. 2.3 Relationships of soil resistance, piledeflection and scour quantity

8

soil or may overestimate the soil resistance for some highly erosive soil.

If a designer is to predict the correct soil response for cyclic loading, ]

additional data on p-y curves are needed along with a sound understanding of

the scour phenomenon in the field.

4[

CHAPTER 3. THE MECHANISM OF SCOUR AROUND THE PILE-SOIL INTERFACE

Scour is the result of the interaction between a fluid-flow field,

cyclic movement of the embedded pile, and the soil particles. The movement

of the pile in response to an applied load is dependent on the soil reaction

against the pile. The soil response is plastic beyond small pile movements,

so if the deflection of the pile is larger than the elastic range of soil re-

action, the plastic deformation of the cohesive soil cannot recover after the

force is released and the water above the mudline will flow into the gap

caused by the soil deformation. When an applied load such as wind or waves

acts cyclically, the water in the interface will be pumped out on each cycle

and both the pressure gradients and fluid velocities will be very high at that

moment.

In the immediate vicinity of the pile the increase of the water

velocity moving into the soil may be high enough to initiate movement of

soil particles. After scouring, the soil particles will remain in the gap

or, if the pumping force is continual and large enough, the soil particles

will be washed out of the interface. Thus, the gap will be enlarged by the

scour with a resulting reduction of soil resistance.

The significant scour mechanism in the interface mentioned above

probably depends on many factors, such as mean diameter of particle, permea-

bility, density of soil, shear strength, speed of cyclic force, elapsed time,

and pile diameter. The experimental work was primarily concerned with the

effects of altering some of these variables.

9

| ~

CHAPTER 4. THE RESEARCH APPARATUS AND PROCEDURES

The scour due to the pumping of the fluid in the pile-soil inter-

face is one of the most complex of fluid-flow problems. A study was under-

taken to find the best way to simulate the scour at the interface. Several

ideas were considered and it was decided to construct a laboratory appara-

tus that would simulate the scour mechanism on a small scale. The appara-

tus is described in this section. It seems reasonable that the scour at

the interface if the pile is in clay will depend not only on the flow char-

acteristics of the fluid causing scour but also on the resistance of the

clay. According to the studies by Sherard 7 , the more dispersive the clay

the more easily it will be eroded by water flow. Because the pinhole test

is a standard test to study the dispersive properties of clay, there is a

possibility that the pinhole test can also be used to evaluate the scour po-

tential around a pile. The use of the pinhole test will be described later.

4.1 PILE-SOIL SCOUR POTENTIAL (PSSP) TEST

The laboratory test was conducted, as shown in Fig. 4.1, in a con-

tainer (1 ft wide, 3 ft long, and I ft deep) made from sheet plastic. An

electric motor with variable speed applied the cyclic movement on the rod

by a connection bar. The 3/4 in. diameter rod was drilled into a soil sam-

ple (6 in. diameter and 7 in. long) and the end of the rod was connected

with the bott6m plate of the sample mold by a hinge to simulate the upper

portion of the pile movement in the field. Water was maintained 2 in. above

the surface of the sample. The displacement of the rod was varied by

changing the eccentricity of a connection bar on the driving disk which

10

0.

. , ,', %' . 3,'3 - V. 3

5 in.

0 0

I0I

I,

7 in.I'

hinge

0 1 1

6 in.

iat

Fig. 4.1 Apparatus used in pile-soil scour potential test (PSSP Test)

12

was mounted on the variable-speed electric motor. The details of some of -uathe components of the test are shown in Fig. 4.2. The test procedure and

measurements are as follows:

A. Sample Preparation

1. Disturbed samples: a representative amount of air-dry, dis-

turbed soil was pulverized using a mortar and a rubber-

tipped pestle, sieved through a No. 40 sieve, and compacted

to the specific density by the AASHO compaction method.8

The disturbed sample with a 6-in. diameter and a 7-in.

height was compacted in 5 layers with a 10-lb hammer using

56 blows per layer. A water content higher than the opti-

mum moisture content was used in compaction in order to'get

a high degree of saturation. After measuring the weight

and water content of the sample, a high back pressure

was applied to the sample until the sample was fully satur-

ated. Then the rod was drilled through the sample and the

sample placed into the testing apparatus.

2. Undisturbed samples: undisturbed samples from the field

were trimmed into the CBR compaction mold for testing. If

3-in.-diameter samples were taken from the field, the com-

paction sample was prepared with the same density as the

undisturbed sample and a hole drilled at the center of the 0specimen to fit the 3-in.-diameter undisturbed sample to

be inserted. The combined soil sample (6-in. diameter) with

a 3-in.-diameter undisturbed core was used for the test.

13

ELECTRIC MOTOR

5"

DISK6

CONNECTION BAR (3/8" 0 x 24" L)

ROD-3/4" x 14" L)

S•"MOLD

(6" € x 7" L)

SEATING PLATE

Fig. 4.2 Details of components of PSSP test

14

The weight of the combined sample was obtained, the rod

was drilled through the undisturbed core, and the sample -

and rod were placed into the apparatus for testing.

B. Test Process

1. A thin disk with a central hole was placed around the rod.

The hole was sized to match the maximum deflection of the

rod at the surface of the soil sample.

2. The eccentric distance was adjusted on the disk of the

electric motor so that the first applied pile deflection

was 0.8 mm.

3. A constant speed of cyclic movement was used, and the

frequency of load application was set at 60 RPM. The test

was run for one hour.

4. A wash bottle was used to suck the suspension into a one-

liter glass beaker. The soil left on the thin disk was

carefully removed by washing and transferred to the same

glass beaker. The weight of the soil was obtained after

oven drying.

5. Steps 2 through 4 were repeated by increasing the deflec-

tion in 0.8 mm increments for each test run.

6. The test was discontinued when the applied deflection

reached one-tenth of the diameter of the rod.

NN_

15

C. Data from Test

1. The amount of scoured soil was measured after oven drying

and the weight was accumulated for each incremental deflec-

tion. A plot was made of the weight of scoured soil vs time.

2. Grain-size curves have been widely used in the identifica-

tion and classification of soils. Because the size of the

individual particles of a soil is an important parameter in

scouring, the grain-size curve of the scoured soil was com-

pared with that of the original soil samples to see if some

of the particles were not scoured. The grain-size analyses

were performed according to ASTM9 standard procedures.

4.2 PINHOLE TEST

7The pinhole test developed by J. L. Sherard measures directly the

dispersibility of compacted fine soils when water is caused to flow through

a small hole punched in a specimen.

A specimen 1.5-in. long and 1.5-in. in diameter was compacted in a

cylindrical mold of Sherard's design. For convenience in using a 3-inch

undisturbed sample without trimming, both 1.5-in and 3.0-in.-diameter cy-

lindrical molds were prepared.

The test specimen and apparatus from Sherard's work are shown in

Figs. 4.3 and 4.4. The general test procedures are:

1. Soil samples must be preserved at the natural water content by

shipping and storing samples in air-tight bags or containers.

.

16

STANDPIPE TO MEASUREWATER PRESSURE K

AT USTRAM ND

PLASTIC TUBE-

PEA GRAVEL

SOIL SPECIMEN

PEA GRAVEL

F-

Figure 4.3 Apparatus for pinhole test ,

0,

NL MVN% VIU WILK-Wv JrwPrU7KUJ KX UKrUY~ Apx NJm X? .w-X3w Ar. 1 .F 11 Al Pvl wp

17

"NIPPLE;'

0: (truncated cone)

ONE LAYER OF -

WIRE SCREEN

CO0M IA CTE ZD

HOLE PUNCHED CLAYIN SPECIM4EN SPECIMEN

TWO LAYERSOF WIRESCREEN

PEA GRAVEL

Fig. 4.4 Section through pinhole test specimen N

18

2. If the material contains particles of coarse sand or gravel,

these are removed by passing the material through a No. 10

sieve (2 mm).

3. The natural water content is measured and the water content is

brought to near the plastic limit by adding the required water

(or by gradually drying, if too wet). All water added should be

distilled water.

4. The 1.5-in.-long disturbed specimen is compacted in the 1.5-in.

cylinder on top of pea gravel and wire screen in five layers

with 16 tamps per layer using a 2-lb hammer. For the testing

of undisturbed soil, the 3-in.-diameter and 1.5-in.-long sample

is extruded from the Shelby tube and trimmed into the cylindri-

cal mold. The gap between the wall of the cylinder and the

specimen is sealed with wax, then the wire screen and pea gra-

vel is placed on the top and the bottom of the specimen.

5. A plastic nipple is pushed into the top of the specimen with

finger pressure and a hole is punched through the nipple with

a 1.0-mm-diameter stiff steel wire (hypodermic needle).

6. After the apparatus is assembled, water is percolated through

the hole in the sample. The heads of water used are 2 in., .

7 in., 15 in., and 40 in. (50 mm, 180 mm, 380 mm, and 1,020 mm)

for time periods of 5 to 10 min at each head. The quantity of

flow is measured continuously with a stop watch and cylindrical

measuring flasks (10 ml, 25 ml, and 50 ml). The color of water

is observed by looking both through the side of the flask and

vertically through the column of fluid in the flask.

o

NJ! Al! J Al! M~r L ! Mi! U2! NJ! , 'i m i W

19

7. At the end of the successive flow tests, the apparatus is dis-

mantled and the soil specimen is extruded from the cylinder

and broken open to examine the size of the hole, which is mea-

sured approximately by comparison with the needle used for

punching the hole.

8. The test is started with a head of 2 in. (50 mm). If no flow

occurs, the test is stopped, the top of apparatus is disman-

tled, and a hole is repunched. Or the first hole is sealed and

a new one is made. This occurs rarely.

9. The principal differentiation between dispersive and nondisper-

sive soils is given by the test results under 2 in. (50 mm) of

head. If the water that flows under 2 in. (50 mm) of head is vi-

sibly cloudy and does not become clearer with time, the specimen is

failing in the fashion typical of dispersive clays. The main

indicator of failure is the colloidal color of the water. Most

dispersive clays erode rapidly under 2 in. (50 mm) of head with

strong color in the water coming through the specimen. Usually

for dispersive clays the flow continuously increases and reach-

es a maximum value limited by the hydraulic capacity of the

equipment in 2 to 5 min of flow (about 1.5 to 2.0 ml/s). The

test is continued for 10 min. Unless the color of the flow

clears substantially, the test is then completed. For the ty-

pical dispersive clay the hole will normally be increased to

about three needle diameters after 10 min of flow. The soil is

classified as highly dispersive (Dl).

~~~~ 01. K-7 .. LS'

20

10. If the water at 2 in. (50 mm) of head has a slight but easily

visible color as seen from the side of the flask at the end of

5 min, the test is continued for 10 min. If water continues

to be colored, the test is stopped. If the rate of flow at the

end of 10 min has not exceeded 0.8 ml/s, and the hole diameter

does not exceed 1.5 needle diameters, the soil is classified as

dispersive (D2). If the test is stopped at the end of 10 min

and the results classified as ND4 and D2, the test should be

repeated with a new specimen to see what happens by raising the

head to 7 in. (180 mm).

11. If the flow under 2 in. (50 mm) of head is clear (or has only

a very slight trace of color as seen from the side of the flask)

at the end of 5 miin, the head is raised to 7 in. (180 mm) and

the test is continued (rate of flow is usually 0.3 to 0.6 ml/s).'

12. If the water continues to flow clear at 7-in. (180 mm) of head,

or if it has only a trace of color as seen from the side of the

flask, after 5 min the head is raised to 15 in. (380 mm) and

the test is continued (rate of flow is usually less than

1.8 ml/s).

13. If the water has color and rate of flow increases rapidly, the

test is stopped. The soil is classified as intermediate (ND3).

The flow at the end of the test will generally exceed 2.5 ml/s,

and the hole size will be larger than 2 needle diameters. 0

14. If the flow is completely clear as seen from the top of the

measuring flask, the head is raised to 40 in. (1,020 mm) after

5 min (rate of flow is usually less than 2.5 ml/s).

0

21

15. If the flow has a slight color or exceeds 3.5 ml/s, the test

is stopped and the soil is classified as nondispersive (ND3).

16. If the flow continues completely clear at a rate usually less

than 4.0 ml/s, the soil is classified as nondispersive (NDi).

There should be no noticeable erosion of the hole in the sample

at the end of the test.

17. If the flow has a bare trace of color or exceeds 5.0 ml/s, the

soil is classified as nondispersive (ND2).

The criteria of steps 9 through 17 are summarized in Fig. 4.5.

'I

:-I~ . ?J......... ... -.--

22

H - 15 in.110 min.

DarkFlowCloudy FlowSlgtyCd Clear Flow

2.0>Q>15(mlQ) .5Q10mi/ ) 1 Q,2. m/s 03( 06mlsDj3l3lDf>Dl 2lDfDl D~

CasfDl Classfy D2 Classify NDl

Finlod pioe diaeete FlDf

Qunite of flo D fQ

Fig.4.5 equeCotsiadclassif icto oftstrslt o

H - n

CHAPTER 5. RESULTS AND ANALYSES 1s

5.1 MEASUREMENT OF RESISTANCE OF SOIL TO INTERFACE SCOUR

Various soil properties such as grain-size distribution, percent and

type of clay, and cohesive shear strength of clay may be selected for study in

regard to scour resistance as measured by the PSSP test. In the experiments

aimed at the development of the method of analysis, four typical soils whose

properties are summarized in Table 5.1 were selected for measurement of scour

resistance. All these samples were prepared by the AASHO compaction method

and the soil density kept as close as possible between specimens. The speed

of the electric motor was maintained at 60 RPM.

The test results are shown in Fig. 5.1. Because the rod deflection

was changed every one hour in the PSSP test, the dry weight of scoured

soil was measured and accumulated every period. The comparison of the re-

lative scour resistance was based on the total quantity of scoured soil

after testing. The more the scoured soil, the higher the scour potential

will be. If the total quantity of the scoured soil is less than 5 gm the

soil is defined as scour resistant. Using this criterion, samples SN2,

SN3, and SN4 are all scour susceptible soils. The soils with the weight

loss more than 20 gm such as the samples SN3 and SN4 seem to have higher

scour potential.

It is apparent that the sample SNI (fine sand) has significantly

more scour resistance than the others. Confirming the scour resistance of

sand, no scour was observed around the piles during the cyclic, lateral-load-

ing test performed at Skyway Bridge, Florida.10 Comparing the soil properties

23

24

1-4 1 cn. 4 en

11 1 r-4 V-4 LM0

.00

v4

04 rI -4 -0 0%4 0% 0

r. 10 Ui cn(a a% ~ p-i

-n "Y4 Ln 0 - - -

E-4

.'-4 P4 -tWJ % U

~~- -44p ~ U

-0 00

'-4 ~ 4-4

oc

25

SN4 3d - 1061b/ft

30

A25 -

0 SN3

= L10lb/ft 3

0

.15 " 5SN2

= 1041b/ft3

10

5

SNI

Yd 1071b/ft3

0 0.8 1.6 2.4 3.2 4.0

APPLIED DEFLECTION AT MUDLINE (mm)

Fig. 5.1 Comparison of scour resistance of various samples

26

of sample SNI and the sample taken from the field of Skyway Bridge, both of

them were found to be cohesionless, medium to fine sand. Their curves of par-

ticle-size distribution are also very close (Fig. 5.2). Thus, it can be con-

cluded that the nonplastic, medium to fine sand is a scour-resistant soil as

indicated by both the PSSP test and field test.

The cohesive soil sample with the highest plasticity had a signifi-

cant gap around the rod during the cyclic movement of the rod. The scour

was clearly visible in photographs (Figs. 5.3 to 5.6). The enlarged gap

from scouring of the cohesive sample SN3 is shown in Fig. 5.7, but no scour

gap was found in the cohesionless sample SNI, shown in Fig. 5.8. Based on

these results, the scour in cohesive soil needs to be carefully considered

in pile design. In order to study the characteristics of soils that are

scoured, the particle-size distribution of scoured soil and scour-resistant

soil are all plotted in Fig. 5.9. Generally speaking, the particles are

smaller for the scoured soil than for the soil in the specimens. This study

seems to show that the particle size is an important factor regarding the

scour resistance of soil.

5.2 MEASUREMENT OF THE INFLUENCE OF VARIOUS FACTORS

ON SCOUR RESISTANCE

Because the scour at the interface of a pile and soil involves the

interaction between fluid flow, cyclic movement of the pile, and soil pro-

perties, varying any one of these three factors will affect the scour

process. The previous study shows that cohesionless sands are not sus-

ceptible to scour, therefore the further studies will be aimed only at clays.

27

,L0i431A A@ MSVOO LN30Udo 2 t $ 3 1 2 33

00

I •

dat LO00•

.4-

T Z!

0 0E3 8

I TI

00rim

LM031 A- MAW LN31

28

Fig. 5.3 View of Manor Clay during testing (Note: photograph taken after10 cycles with a displacement of the rod of 0.8 mm)


29

Fig. 5.5 View of Manor Clay during testing (Note: photograph taken after300 cycles with a displacement of the rod of 0.8 m)

I 4,.


%

30

• , -,°

Fig. 5.7 View of scour gap after testing (Note: photograph taken fromsample SN-3 after testing)

Fig. 5.8 View of non-scour gap after testing (Note: photograph taken fromsample SN-I after testing)

31

0 0,

-,440 0

W-4

-T 0

I a) I Q)W

000

L~'LO

Ow -41- -

L2S SJ 0-Jtf C0-u

o-r

4.

2r0

Q- 0

,o Iv 0

- -4!I -1

0 0

oeU

INO13 AMCflLN~

32

The experimental program included changing some of these variables for de-

tailed study as discussed in the following sections.

5.2.1 EFFECT OF SOIL PROPERTIES ON SCOUR RESISTANCE

A. Sample Disturbance

The cohesive shear strength generally is reduced by soil

disturbance which is inevitable in pile driving. Because the

Manor clay (sample SN4) with high sensitivity, fissures, and

cementation is easily disturbed, the undisturbed Manor Clay

taken from the field was used in this study (Fig. 5.10). The

influence of soil disturbance on scour resistance is shown in

Fig. 5.11. The scour resistance of the undisturbed sample is

significantly higher than that of the compacted sample (fully

disturbed). The decrease of the cohesive shear strength due to

the destruction of cementation is a possible explanation for ,

this effect.

B. Soil Density

Generally, as the density of a clay increases there is an

increase in the shear strength and an increased scour resistance

would be expected. Figure 5.12 shows the relation of soil density

to the scour resistance of Manor clay. The scour does not change

greatly with different densities and water contents for both sample

SN2 and SN4. However, the lower percentage of clay in sample SN2

has some effect on scour resistance at the higher density.

Lk

33 I

Fig. 5.10 View of 6-in.-diameter undisturbed Manor Claym

34

30 .UCompactedSample

25

,4

0 2

.. .....

10 . . .. ... .

15 Sampl...

0

0. 1.10432 .

Fig........5.11..............................nscou Ueitndeisfurbed

N . ,

35

50

40 Manor Clay (SN-4)

0 o

30

o 20-4

02

0

oU ClayeI SI (SN,2

10

85 90 95 100 105 110

Dry Density (lb/ft

Fig. 5.12 Influence of soil density on scour resistance

36

5.2.2 FREQUENCY OF PILE MOVEMENT

Because the velocity of water being squeezed from the gap in

the soil around a pile will undoubtedly influence the scour, the in-

crease of the frequency of pile movement should have an influence on

the rate of scour. On the other hand, the more the number of cycles,

the more interactions between pile, soil and fluid there will be. It

is believed that there is an increase in scour if the rod frequency

is kept at 60 RPM and the number of cycles is increased. Figure 5.13

shows that the scour of Manor clay varies with the pile frequency.

The more scour at higher speed of pile movement can be attributed to

both the increase in cycles and in fluid velocities.

A. Number of Cycles

The scour is caused by cyclic movement of pile; thus, the

number of cycles seems to be an important factor in scour process.

The dry weight of scoured soil is plotted versus the loga-

rithm of number of cycles in Fig. 5.14. It may be seen that the

scour is influenced both by the frequency and by the number of

cycles. For a given deflection of the rod there must be number of

cycles at which no more scour would occur but that equilibrium

condition could occur at a very large number of cycles.

B. Fluid Velocity

The tractive force of fluid flow causes the soil particles

to be loosened and eroded on the soil boundary. Because the

magnitude of the tractive force is proportional to the fluid

velocity, a knowledge of fluid velocity is necessary in making

37

o 50

1-4o40

3~d= i00 ib/ft 3

40 27,000 cycles

U,

44I0

00

18,000ccle cyc6lb/f27,000 cycles

0 24,000 cycles4J

= 20

U 0 c es 12,000 cycles

10 0 9,000 cycles

0 20 40 60 80 100

Frequency (RPM)

Fig. 5.13 Influence of pile frequency on scour resistance of Manor Clay

NO _

38

Cycles

100 1,000 10,000

25 P- ile Frequency

90 RPM

0 60 RPM

S 20 RPM20

,.,4

0 15

0

00o 10-

4-

0

-4y

Fig. 5.14ao Influence lyof the number of cycles on scour resistance of

Mao3Ca

% V

39 .

analytical studies of the scour of soils.

As a means of developing an expression for the velocity of

flow from the gap around a pile (or from the gap around the rod

in the PSSP test), it is assumed that the pile or rod acts like

a plunger in a cylinder. The further assumption is made that

the average area of the gap is equal to one-half the area at the

mudline. With these assumptions, the fluid velocity v in the

PSSP test can be estimated simply as follows:

A L-fr

L f ..............A A 2

in which L = the length of the scour opening (length of rod);

V = the volume of scour opening;

A - the area of cross section of scour opening at

mudline, and

f = the time required for the scour opening to be

Aclosed.

L

L ""_..Scour hole i}:

I

II

40

Figure 5.15 shows the influence of fluid velocities on scour

at different numbers of cycles of loading for Manor Clay. The

soil properties are probably influenced by the water at the pile-

soil interface and vary with time in a complex manner. The swell-

ing of cohesive soil generally causes a decrease of the interpar-

ticle bonding force and thus the critical shear stress required to

promote scour is reduced. If the number of cycles is as low as

1800, the elapsed time is 1.5 hours at a frequency of 20 RPM, but

only 20 minutes duration for 90 RPM. The variation of soil pro-

perties with time could be important and perhaps the dominant fac-

tor in scour; thus, the influence of fluid velocities may not

easily be detected. The decrease of the interparticle bonding

force due to a longer duration of testing is a possible reason to

explain the more scour at a lower frequency in Fig. 5.15.

However, when the number of cycles is large and the duration

of testing is long enough so that the soil properties at the in-

terface are not likely to be changing much, the effect of fluid

velocity is more apt to appear. Curves with the number of cycles

over 7,200, where the duration of testing is longer than one hour,

can be seen in Fig. 5.15. Samples acted upon by the higher fluid

velocities seem to have a significant decrease in scour resistance

at the same number of cycles. The influence of the variation of

soil properties with time, mentioned before, seems not to be very

%9.% % le

0 7w %J

41_7

25

20 t=2 hrs

-4080

0

0 t

t=0.7 hrs

5 -N=1,800

tO. 3 hrs

0 1

5 10 15 20 25

Fluid Velocity (ft/mmn)

Fig. 5.15 Influence of fluid velocity on scour resistance of Manor Clay

42

significant and may be overshadowed by the increase of the fluid

velocity.

Generally speaking, if the soil properties remain contant

during a test, the high fluid velocity undoubtedly produces a

greater scour than a low fluid velocity.

5.2.3 PILE DIMENSIONS

From Eq. 1, the fluid velocity is only a function of the depth

of the scour hole and the pile frequency. However, the large diame-

ter pile has more interface for scouring and probably the depth of

the scour hole will indirectly be a function of the pile diameter.

If so, the fluid velocity will be also affected by the pile diameter.

A comparison of the scour using different sizes of rods in the

laboratory test is shown in Fig. 5.16. Because the PSSP test was

not performed on a full-size pile and the length of the rod is con-'

stant, the rod diameter may not show any significant influence on the

depth of scour. Without any change in scour depth, the interaction

area of scour should be proportional to the rod diameter. Figure

5.16 shows that the relation between the curves is 3/4 to 1, which

is precisely what it should be at the constant scour depth. The

increase of the interface area for the large diameter rod could be

the only reason to cause the more weight loss in PSSP test.

5.3 EQUILIBRIUM STAGE OF SCOUR AROUND THE PILE-SOIL INTERFACE

According to observations in the field, the scour process decreases

with the increase of duration under the constant cyclic movement of piles.

U0

43

OD=1"

30

ODin3/4"

25

-4

M 20

V)

0U

4

w~.

10

5d

0

0. .6243. .

Aple Dfetona ulie(m

Fi.51 nlec fpl imtr nsorrssac fMnrCa

44

If the amplitude of pile movement is held constant by a layer of strong

soil below the mudline, an equilibrium condition should develop at which

little or no additional scour will occur.

Scour equilibrium is achieved when the volume of material removed by

the water is decreased to a very small amount. At that moment, the scour

opening is somewhat larger than would be indicated by the amplitude of

cyclic pile movement.

The observations made during this study were that rapid and dramatic

scouring normally took place during the first hour of each test, but the

time to reach scour equilibrium appeared to be longer than one hour. It

is apparent that the boundary conditions and many other factors affect

the equilibrium stage. At present, there is no way to predict whether

or not there is any similarity between the behavior of full-sized piles

at offshore sites and the observations made in the laboratory. As noted

earlier, the thrust of this study was to develop a simple laboratory test

for ascertaining the scour potential of various soils.

The tests that involved the change of frequency with constant dis-

placement show that the low speed of rod movement gave a slow scour pro-

cess. According to the laboratory tests, the duration for the equilibrium

stage at 20 RPM was about twice as long as that at 60 RPM (Fig. 5.17).

Because the fluid velocity varied with the speed of pile movement, the

quantity of scoured soil after reaching the equilibrium stage at 60 RPM

is significantly higher than that at 20 RPM.

The equilibrium stage was not found in three hours of laboratory

testing when the pile frequency was kept at 90 RPM as shown in Fig. 5.17.

0 r 0

. m"M"an aB a ra Z l r' a ,r ,d - .~ . r.. ,j. • VuUuuv4WVW "V V. P JWV 71 W'J r'.CW f 7*d 7. 4 ma '4 " vJ' al & J' W

45

o20

15-0U

10

5

U )

0

0.5 1.0 1.5 2.0 2.5 3.0

z. 10

00

m4 5

0.5 1.0 1.5 2.0 2.5 3.0

Duaio5hor

Fig.~ ~ ~ ~ ~ ~ ~ ~ ~~~ Pl 5.7Rltosi ewe cu aeaddrto frqestncyoManor 9CRay

16. 604RPM

WN WV N- -6wwi . Tu W _ . K WV. 4V jr.. VV W,V '.1 5 X~ ~ W ~ wq ~~.-

46--

In order to gain further information the duration of testing was increased

to 5 hours. However, the scour rate was still measurable at the end of test-

ing as shown in Fig. 5.18.

Although the equilibrium stage sometimes cannot be observed properly

in the laboratory due to the limit of testing time, the data do clearly show

that the scour rate was gradually decreasing after one hour of testing.

Based on these observations, the scour process can be divided into two stages

in the PSSP test: stage one is called the main-scour stage and is where

scour is occuring rapidly with a high scour rate, and stage two is called the

residual-scour stage and is where the scour rate has become relatively con-

stant. Generally speaking, the residual-scour stage, with a very low scour

rate, has less effect on the soil resistance because the scour hole is al-

ready larger than the amplitude of pile movement after the main-scour stage.

If the drive speed is maintained at 60 RPM, the relatively large rod

deflections will cause initially a relatively high rate of scour. Figure

5.19 shows the relationship between scour and duration of testing for

cases where rod movements and frequency were kept constant. The time to

reach the equilibrium stage for pile deflection larger than 1.6 mm is not

shown by Fig. 5.19, but the scour processes for different rod deflections

seem to be very similar. Although the quantity of scour still increased

after one hour, the main scour was seen to be fully completed. The scour

process may be faster for the smaller rod deflection at a constant fre- %

quency. •

47

25

-. 20

-,4

0

si 150

U)

4J

-4

0

0

W 5

$44

0

1 2 3 4 5

* So

105

O -. 5

Fig. 5.18 Relationship between scour rate and duration of testing of Manor

Clay under constant pile deflection (1.6 mm) and pile frequency (90 RPM)

1 20

0i

150U

0. .0 10. . 30.

'lo-

Pile Deflection

0.-3.2m

0

0.5 1.0 1.5 2.0 2.5 3.0

15

0

S10

Duration (hour)

Fig. 5.19 Relationship between scour rate and duration of testing of ManorClay under constant pile frequency (60 RP)

02.i

49

5.4 COMPARISON OF RESULTS FROM PINHOLE TEST AND FROM PSSP TEST

In the last few years it has become more clearly understood that

there are in nature certain dispersive clays10 that are highly erodible.

The fluid running through the dispersive clay carries a cloudy colored

suspension of colloidal particles. The principal difference between dis-

persive clay and ordinary erosion-resistant clays is the nature of the

cations in the pore water. Dispersive clays have a preponderance of

sodium, whereas ordinary clays have a preponderance of calcium and magne-

sium cations in the pore water. The studies based on the pinhole test by

Sherard show that dispersive clays erode rapidly in the pinhole

test. Conversely, essentially all the low sodium clays, previously desig-

nated as "ordinary, erosion-resistant clays", do not erode in the pinhole

test. If the soil around the pile is dispersive clay, the scour resistance

must be very low.

The pinhole test is not designed to be used as a quantitative test

for measuring rate of scour as a function of the velocity of flow water,

but it does serve as a good method of identifying dispersive clays.

Pinhole tests were performed on specimens SN-2 to SN-4 and the test

data are presented in the Appendix. Comparisons between results from the .

pinhole test and the PSSP test are listed in Table 5.2. It can be estab-

lished conclusively that dispersive clay is not a scour-resistant soil,

but the high scour-potential soil found in the PSSP test may or may not

be a dispersive clay.

50

TABLE 5.2 SUMMARY OF TEST RESULTS FROM PINHOLE TEST AND PSSP TEST

Classification of Soil

Test Sample

Pinhole Test PSSP Test

SN-1(fine sand) Scour Resistant

SN-3 Signiticant Scour(silt) Susceptibility

SN-2 Intermediate Scour(clayey silt) Non-Dispersive Susceptibility

SN-4 Significant Scour(Manor clay) Non-Dispersive Susceptibility 1

51

5.5 DATA INTERPRETATION

According to the study presented in this chapter, the weight loss in

a given period of time is a function of the following variables:

C : soil properties (shear strength, unit weight, permeability,

grain-size distribution, cations present).

N : number of cycles

v : fluid velocity

t time

D pile (rod) diameter

h water head

d : cyclic deflection of pile (rod)

If the pile diameter and water head are held constant, these vari-

ables can be related by a simple equation of the form

S = 4(C, N, v, t, d) ................................ (2)

where S - the dry weight of scoured soil. Because the fluid velocity is

not easy to determine in the field, the frequency of loading can be used

to relate the fluid velocity and the number of cycles. Then Eq. 2 can be

written in the following form by assuming these variables are independent

of each other

S _ I f, . tn dp ............ (3)

in which m, n, p are the exponents for frequency, time, and rod deflec-

tion; I is a proportionality constant related to the soil properties. I

52

It is convenient to define I as the scour index of soil so that the

relative scour for different samples can be compared. To determine the

exponents in Eq. 3, the relations of log S versus log f, log S versus log

t, and log S versus log d from Manor Clay were plotted on Figs. 5.20, 5.21

and 5.22, respectively, from which the exponents m, n, and p can be easily

derived.

According to these analyses, initial values for the parameters in

Eq. 3 have been determined:

Si I•. fO.6 0.55 0.6

where f = loading frequency, cycles per minute

t duration, hrs

d - rod deflection, mm

I= scour index of soil .u.

S - dry weight of scoured soil, gm

The scour index I of remolded Manor clay computed from Eq. 3, based

on experimental data, is about 0.56. Figures 5.23 and 5.24 show the compa-

risons of experimental curves and the curves calculated using these

parameters and a reasonable fit is seen.

Furthermore, the scour indexes of the other soils were computed from

Fig. 5.1. Table 5.3 presents a summary of the scour indexes determined

for the various soils. The differences in the indexes are instructive in

reflecting the relative susceptibility to scour of the four soils that

were studied. Table 5.3 shows that the scour indexes for clays are signifi-

cantly higher than that of the cohesionless soil. As mentioned in this

" :". ,r . ... 5 s' .

53

100

Duration (hours)

V 3.050 0 1.5

A 1.00 0.5

20'-4

0

S100Q

0

0-

5

:33

2

SI . , I ,. I I . , I . ..

110 20 50 100 200 500 1000

Pile Frequency (RPM)

Fig. 5.20 Relationship of pile frequency and dry weight of scouredsoil (Log Scale)

54

100 I - ~Frequency (RPM)

V 90

50 0 600 20

200

0

Q 1000

In

0

20

0.1 0.2 0.5 1.0 2.0 5.0 10.0

Time (hour)

Fig. 5.21 Relationship of duration and dry weight of scoured soil(Log Scale)

55

100

Duration (Hours)

50 7 2.5

1.5

0 1.0

0 0.5

-420

*0

0

010

0

o5-,4

2

1 2 5 10 20 50 100

Pile Deflection (mm)

Fig. 5.22 Relationship of pile deflection and dry weight of scouredsoil (Log Scale)

rww'wAMAUMP'I A

56

Pile Frequency Experimental Data Fitting Data

90 RPM

60 RPM 0

3020 RPM 0

,25

o 20U

0

39 15I.'O

10U

000

00.5 1.0 1.5' 2.0 2.5 3.0

Duration (hour)

Fig, 3.23 Comparison of experimental data and fitting equation for ManorClay at constant pile deflection (1.6 mm)

40

57

Pile Deflection Experimental Data Fitting Data

4. 00m 0

30 2.40 A

1.60 mm 0

25

0u20

~00

00

'Oee0.

4

5 /

0

o.5 1.0 1.5 2.o 2.5

Duration (hour)

Fig. 5.24 Comparison of experimental data and fitting equation for

Manor Clay at constant pile frequency (60 RPM)

IK-1

58

TABLE 5.3 SUMMARY OF SCOUR INDEXES OF SAMPLES

Sample Scour Index I

SN-i 0.06

SN-2 0.24

SN-3 0.40

SN-4 0.56

IS * ** "

I. %-

59.

chapter the scour in the pile-soil interface occurred when the soil was

molded away from a pile. Because the cohesionless sand was not involved in

the "molding away", the sand will not be scoured like clays, but there are

some fine particles in the sand that can be pumped out through the voids by

the cyclic action of piles. Generally this quantity is too small to be

counted.

'N

CHAPTER 6. RESULTS FROM TESTING NATURAL SOILS

The study reported in Chapter 5 showed the natural soil to have a well-

defined scour potential, and it was decided that it would be desirable to

test other natural soils. The scour indexes determined from the PSSP test

can possibly be of benefit to practicing engineers. Because 6-in.-diameter,

undisturbed samples were not available, 3-in.-diameter, undisturbed samples

were prepared for testing as described in Chapter 4.

Undisturbed samples of Manor Clay, North Sea Clay, and Sabine Clay

were available and their general characteristics are listed in Table 6.1.

The Manor Clay has the highest plasticity index (PI) of the three and is

classified as CH according to the Unified System, a clay with high plasti-

city. The Sabine Clay is just inside the CH range and probably should be

classified as CL-CH, a clay with a medium plasticity. The Sabine Clay had

the highest natural water content of the three and the lowest density

(100 lb/ft 3). The North Sea Clay had a sand content of 28% and is classi-

fied as a low plasticity clay, a sandy or silty clay. Each of these samples

consisted of fine grains and were thought, prior to testing, to be scour sus-

ceptible.

The gradation curves for the three soils are shown in Fig. 6.1. S

The comparisons of scour potential between Manor C lay, North Sea C lay,

and Sabine Clay are shown in Fig. 6.2. According to their scour indexes

(about 0.27 to 0.48), they all have noticeable scour potential if compared

with the values shown in Table 5.3. The variation as shown by upper-bound

and lower-bound curves for each soil in Fig. 6.2 reflects the differences in

60

% A V

61

0 J -13: -e 0 Lrl

C 0z u

zgo-4 n IT %

0 -

w_

~~C4Wf,~

w

E-1 ~ .U

0

0.0

1-4 01 41

04

0 cu

000

E 44

00

0 a,

62

LMOS~ AS MWOO LW*WUd

FL2.. * 8 RL I

II -OA 0

u

-4 0

0 0cn 0

0 00 0

tun, 4 44

0

:P, 0

en 0

0 cc 0

so~

M C6

, a

A MM LM

63

Soil Upper Bound Lower Bound Scour -Index-

Manor 00 10.27Sabineea A A 0.36

LNorth Se 1 0.48

30

25U

/A

0 20/

00

:x 15

10/

00

0.8 1.6 2.4 3.2 4.0

Applied Deflection at Mudline (mm)

Fig. 6.2 Comparison of scour resistance of Manor Clay,North Sea Clay, and Sabine Clay

64

the natural soils (samples from different depths were tested) and the scour

index was taken average between the curves. It is apparent that the scour

potential of the North Sea Clay is significantly higher than the others.

There are many unknowns about the influence of soil properties on scour

resistance, but generally the higher PI value will mean a higher cohesive

strength. Sands do not exhibit much scour because water can flow through

the pore spaces. Clays with a greater cohesive strength should be more

scour resistant than those with lesser cohesive strength. Because all these

samples are cohesive fine soil (Fig. 6.1), the lower plasticity of North Sea

Clay could be the principal explanation for its higher scour potential.

The general characteristics of scour can be seen from the comparisons

in Fig. 6.2 for Manor Clay, North Sea Clay, and Sabine Clay. The scour

indexes do serve as a good indication of the results of the tests. However,

further studies are necessary before the scour index can be integrated in

the design process for offshore structures.

S!

CHAPTER 7. RECOMMENDATIONS -s

For the present, it is recommended that the PSSP test be employed in

the laboratory to investigate the scour potential of various soils.

With regard to details, it is recommended that the following proce-

dures be adopted.

1. The test arrangement should be as shown in Figs. 4.1 and 4.2 with

the soil mold having a diameter of 6-in. (15.2 cm) and a height of 7-in.

(17.8 cm).

2. The rod used should be steel pipe with 14-in. (35.6 cm) length and

3/4-in. (1.9 cm) diameter and hinged at the bottom of the mold.

3. The frequency of load application should be 60 RPM.

4. The initial deflection at the surface of the sample should be h

1/32-in. (0.8 mm) and subsequent deflections should be in increments of

1/32-in. (0.8 mm).

5. The duration should be 1 hour for each deflection and the scoured

soil collected every one hour.

6. The test is to be continued until the applied deflection reaches

5/32-in. (4.0 mm). The dry weight of the scoured soil is accumulated for use

in comparing the relative scour potential of various soils.

I

CHAPTER 8. SUMMARY AND CONCLUSIONS

This report presents a study of the scour potential around the pile-

soil interface. A laboratory test (PSSP test) was developed and studied

thoroughly. The PSSP test was successfully applied to evaluate the scour

potential of several soils.

From an engineering standpoint, the aim of this investigation was

to provide a means by which an engineer can make reliable esti-

mates of the scour potential of various soils. If additional data become

available in time, it may be possible to use the scour index obtained by

the PSSP test in making quantitative predictions of p-y curves. The

complexity of the total problem eliminates a simple and quick solution.

A satisfactory answer may require the measurement of scour-resistance

properties of a soil, the variation of shear strength with time, patterns

of fluid flow around the pile-soil interface, depth of scour opening, and

the reduction of soil resistance on the pile. All of these problems are

worthy of further investigation in order to get a reliable evaluation of

scour potential.

The following conclusions were drawn from the experimental study of

scour around the interface of marine piling:

1. The particle size is an important factor in scour resistance.

Non-plastic, coarse to fine sand seems to be a scour resistant soil.

Cohesive soil with finer particles is more susceptible to scour. "

2. The scour is affected by a variation of soil properties. The com-

pacted Manor Clay shows higher scour potential than the undisturbed Manor

66

patted

67

Clay. The North Sea clay was more susceptible to scour than natural soils

with a higher clay content.

3. The speed of rod movement has a significant influence on scour.

The soil seems to be less scour resistant at higher frequencies of loading.

4. The dispersive clay defined by the pinhole test is not a scour

resistant soil, but the high-scour-potential soil found in the PSSP test

may or may not be a dispersive clay.

5. The equilibrium stage of scour is defined as that at which little

or no scour is occurring with increased cycles of deflection. The scour

process will reach the equilibrium stage finally if the cyclic deflection

of pile is held constant by underlying layers. The time to reach the

equilibrium stage varies with the boundary conditions.

6. Using the scour index from the PSSP test seems to be a feasible

method for the evaluation of sour potential and is worthy of further study.'

V

REFERENCES

I. Chow, W. Y., and Herbich, J. B., "Scour Around a Group of Piles,"Offshore Technology Conference, Paper No. 3308, 1978.

2. Machemehl, J. L., and Abad, G., "Scour Around Marine Foundations,"Offshore Technology Conference, Paper No. 2313, 1975.

3. Niedoroda, A. W., Dalton, C., and Bea, R. G., "The Descriptive Physicsof Scour in the Ocean Environment," Offshore Technology Conference,Paper No. 4145, 1981.

4. Reese, L. C. & Matlock, H., "Non-Dimensional Solutions for LaterallyLoaded Piles with Soil Modulus Assumed Proportional to Depth,"Proc. 8th Texas Conference S. M. and F. E., Spec. Pub. 29, Bureauof Engineering Research, The University of Texas at Austin, 1956.

5. Matlock, H., "Correlations for Design of Laterally Loaded Piles inSoft Clay," Proc. 2nd Offshore Technology Conference, HoustonVol. I, 1970.

6. Reese, L. C.,, Cox, W. R., and Koop, F. D., "Field Testing and Analysisof Laterally Loaded Piles in Stiff Clay," Proc. 7th OffshoreTechnology Conference, Houston, Paper No. 2312, 1975.

7. Sherard, J. L., Dunnigan, L. P., and Decker, R. S., "Identifying Dis-persive Soils," Journal of the Geotechnical Engineering Division,ASCE, Vol. 102, No. GTI, 1976.

8. Bowles, J. E., Engineering Properties of Soil and Their Measurement,McGraw-Hill, New York, 213 p., 1987.

9. American Society of Testing and Materials, Annual Book of ASTM Stan-dards, Part 19: Soil and Rock; Building Stones, 1981.

10. Reese, L. C. and Long J. H., "Analysis of Two Drilled Shafts Subjectedto Lateral Load, Sunshine Skyway Bridge," Geotechnical EngineeringResearch, The University of Texas at Austin, 1982.

11. Perry, E. B. and Postol, G. J., "Dispersion Characteristics of Clayfrom Mushroom Cave, Meramec Park Lake Project, Meramec River,Missouri," Dispersive Clays, Related Piping, and Erosion in Geote-chnical Projects, ASTM STP 623, J. L. Sherard and R. S. Decker,Eds., American Society for Testing and Materials, 1977.

12. Sherard, J. L., Dunnigan, L. P., and Decker, R. S., "Identificationand Nature of Dispersiv? Soils," Journal of the GeotechnicalEngineering Division, ASCE, Vol. 102, No. GT4, 1976.

68

16-A

APPENDIX

Pinhole Test Data

69

FMU UW Uw w WWA

70

PINHOLE TEST DATA

Pinhole Test No. 1Date: May 30, 1982

Sample No. SN - 3 Page: 1

Compaction Characteristics '( -106 lb /f t 3 Specimen After Test:

Water Content 29.47.

Distilled Water Added: V or 3___Yes No

Curing Time: 4 hours

Flow Started On 1st Trial.

CokFlow Color From Side ParticlesClock Head Rate - - -Falling

- - -o Remarks

8:00p in. ml sec M "4 WA -, .A.CC r4G W ve -4 0e 0 4 0 GA A

45'30'1 2 1001 76 V V

47'30"* 2 100 72 V V

51'30" 2 100 73 JV v

55'30" 2 100 73 V v

58'30" 2 100 74 V v

(90) 2 100 76 V V Stop Test02'30" - -40 - .

_____ I__ _

-1 1 -, -1 1 f 1 F

71

PINHOLE TEST DATA

Pinhole Test No. 2 Date: June 1, 1982


Compaction Characteristics f(-94.8 lb/ft3 Specimen After Test:

Water Content 34.6% II

II

Distilled Water Added: V or_ _

Yes No

Curing Time: 4 hours,I


Flow Color From Side ParticlesClock Head Rate Falling

0- 4 RemarksE-40 0

"4 .a =-.4 CL 0a w1:o pt in. ml sec w -A-0 w f E!cc40 0 1U.4 0O 4 0p- 0 0 w

40'00" 2 100 87 V V

42'00" 2 100 90 V V V

44'00" 2 100 88 V V V

47'00" 2 100 87 V v v

51'00" 7 100 49 V V V

53'00" 7 100 56 V v v

55'00" 7 100 54 V V v

57'00" 754 V V V'4

wmvuvu.wLd 41rLrUVL ~ nA.r .' ' 5M-1

72

PINHOLE TEST DATA

-S



Compaction Characteristics f 4 =94.8 lb/ft Specimen After Test:

Water Content 34.6%

Distilled Water Added: V or ,,____Yes No -- i- 1.0 to 1.5 mm

,!

Curing Time: 4 hours"


Flow Color From Side ParticlesClock Head Rate FallingTime

-0 Remarks

.6))

2:00pm in. ml sec W 4 -0 w w s 0 e W ) C 3 >0 4 W 0~. A , 0- 0 0) 0)

02'00" 15 100 44 V V V

04'00" 15 100 44 V V V

06'00" 15 100 43 V V V

08'00" 15 100 45 V V V I

12'00" 40 100 34 V V V

14'00" 40. 100 351 V V 7 ---v

16'00" 40 100 35 V V V

18'00" 40 100 34 V V V Stop Test

ji

73

PINHOLE TEST DATA



Compaction Characteristics Y'( -98.7 lb/ft3 Specimen After Test:

23%Water Content __

Distilled Water Added: V or I

Yes No IIYesNo *"1.0 to 1.5 mm

Curing Time: 1 day ,,


Flow Color From Side ParticlesClock Head Rate FallingTime

o Remarks

3:00pm in. ml sec -4o - >

"0 ,W ccr "4 0 0n to .> U= U = v L) z

47'00" 40 900 197 V V

53'00" 40 900 196 V V v

[4 :OOpm401,001 -40 -00 252 v v

08'00" 40 1000 254 V V V 0

25'00" 40 1000 264 V V V

35'001 40 1000 257 V V V

50'00" 40 1000 278 V V V - - ]

58'00" 40 1000 276 V V V Stop Test %,

74

PINHOLE TEST DATA

Pinhole Test No. 4____________ Date: June 5, 1982

Sample No. SN -2 Page: 2

Compaction Characteristics rd in1O5 lb/ft 3 Specimen After Test:

Water Content 19.2%

Distilled Water Added: V or ____

Yes No 1.0 to 1.5 mm

Curing Time: 12 Hours '

Flow Started On Trial.

Clock Flow Color From Side Particles

Time Head Rate -Falling

>% >1 Remarks

I - ) W)

W~ "O. ).4 M 0 cc0 4)>2:00pm in. ml sec ,.1 V W MC 0) W C 3 cc

V -4C 0 Ov"4 4 0 Q) Q)

08100" 40 900 210 v V

1b'00"1 40 900 210 V V V

30100" 40 900 2151 V V V

245O00m 40 900 220 V V V

5:00pm 4 0 1 Stop Test00130"14 9028

7 P.

f/LA)FEwjh

171- moor

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SOILS..(U) mhhhhhl--mIl mmhhhhhhhhhhhl hlflflflflflflfll ... · of soils at the interface of piles that support offshore structures. These soils are sub- ... CHAPTER 7. RECOMMENDATIONS